CIRCUMBASINAL OUTCROP PERSPECTIVE OF THE MARCELLUS “SHALE”, APPALACHIAN BASIN
Antiquated, inaccurate stratigraphic interpretations mask true stratigraphic relationships, and obstruct clarity in Marcellus analyses. As defined in NY, the Marcellus comprises strata between the Onondaga Limestone (below) and the Skanelateles Formation (above). Throughout the basin these form two major transgressive-regressive cycles/3rd order depositional sequences (Union Springs & Oatka Creek units of NY). In basinal areas, Marcellus strata occur as two black shales bounded by more calcareous mid-Marcellus strata (Cherry Valley & Hurley +/-Stony Hollow Mbrs. of NY; =Purcell Mbr., PA, MD, VA, WV). In proximal facies adjacent to the Acadian orogen, correlative mid-Marcellus sandstones (Turkey Ridge Mbr., PA) bound lower and upper black shale tongues. Proximal upper Marcellus strata comprise a progradational sequence of basinal black shales to nearshore and terrestrial facies (Mount Marian, lower Ashokan Fms., eNY). On the distal cratonic basin margin, lower to middle Marcellus-age shallow ramp carbonates (Delaware Fm., OH) were deposited; a tongue of the lower black shale, formed during maximum transgression of the lower sequence, is locally developed.
Basal transgression of the lower (Union Springs) sequence begins in the upper Onondaga Ls. The thin Hurley & Cherry Valley Mbrs. and equivalents basinwide mark the base of the upper (Oatka Creek) sequence. Locally, under- and overlying sequences may also become OC-rich, mimicking OC-rich Marcellus facies. In some areas of VA and WV, as many as 13 Lower to Middle Devonian sequences may occur largely as black to dark gray clastic mudrocks (Needmore & Millboro Fms.).